WASHINGTON — Battling brain cancer, Sen. John McCain on Thursday promised to return to work, leveling fresh criticism at the Trump administration and aiming a good-natured dig at Republican and Democratic colleagues who were jolted by news of the six-term lawmaker’s diagnosis.

“I greatly appreciate the outpouring of support — unfortunately for my sparring partners in Congress, I’ll be back soon, so stand-by!” McCain said in a tweet.

The 80-year-old McCain, the GOP’s presidential nominee in 2008, was diagnosed with glioblastoma, an aggressive type of brain cancer, according to doctors at the Mayo Clinic in Phoenix, who had removed a blood clot above his left eye last Friday.

The senator and his family are considering further treatment, including chemotherapy and radiation, as he recuperates at his home in Arizona.

Showing no signs of stepping back from political and national security battles, McCain issued a blistering statement through his office criticizing the Trump administration over reports that it was ending a program to assist Syrian opposition forces fighting the government of Bashar Assad.

“If these reports are true, the administration is playing right into the hands of Vladimir Putin,” said McCain, chairman of the Senate Armed Services Committee. “Making any concession to Russia, absent a broader strategy for Syria, is irresponsible and short-sighted.”

More significantly, McCain’s absence is forcing Majority Leader Mitch McConnell, R-Ky., to delay action on health care legislation. Republicans need his vote in order to move forward on repealing and replacing President Barack Obama’s law.

Meantime, prayers and words of encouragement multiplied on Thursday from presidents and Senate colleagues past and present.

“I called Senator John McCain this morning to wish him well and encourage him in his fight. Instead, he encouraged me,” said former President George W. Bush, who prevailed over McCain for the GOP presidential nomination in 2000. “I was impressed by his spirit and determination.”

Former Sen. Bob Dole of Kansas said: “Having known John for many decades, I am certain that he is as tough as they come ’ if anyone can defeat this, it’s him. John is a true American hero.”

McCain’s closest friend in the Senate, Lindsey Graham of South Carolina, said that they had spoken by telephone Wednesday night and that the diagnosis had been was a shock to McCain. He said that “woe is me” is not in his DNA. “One thing John has never been afraid of is death,” said Graham, who said he expects McCain to be back at the Capitol.

According to the American Brain Tumor Association, more than 12,000 people a year are diagnosed with glioblastoma, the same type of tumor that struck McCain’s Democratic colleague in legislative battles, the late Ted Kennedy of Massachusetts. The American Cancer Society puts the five-year survival rate for patients over 55 at about 4 percent.

McCain, a former combat pilot, has a lifetime of near-death experiences ’ surviving a July 1967 fire and explosion on the USS Forrestal that killed 134 sailors, flying into power lines in Spain, being shot down in October 1967 and falling into Truc Bach Lake in Hanoi, and going through 5 1/2 years in a North Vietnamese prison.

“The Hanoi Hilton couldn’t break John McCain’s spirit many years ago, so Barbara and I know ’ with confidence ’ he and his family will meet this latest battle in his singular life of service with courage and determination,” said former President George H.W. Bush.

Commenting on both McCain and the response, Sen. Dick Durbin, D-Ill., said, “The outpouring of bipartisan respect and love for John McCain as he faces this cancer battle reminds us that after all the meanness there is a human side to politicians. Count this Democrat in John McCain’s corner.”

In the past, McCain had been treated for melanoma, but this primary tumor is unrelated. Doctors said McCain is recovering from his surgery “amazingly well” and his underlying health is excellent.

With his irascible grin and fighter-pilot moxie, McCain was elected to the Senate from Arizona six times, most recently last year, but was twice thwarted in seeking the presidency.

An upstart presidential bid in 2000 didn’t last long. Eight years later, he fought back from the brink of defeat to win the GOP nomination, only to be overpowered by Obama. McCain chose a little-known Alaska governor as his running mate in that race, and helped turn Sarah Palin into a national political figure.

McCain returned to the Senate, determined not to be defined by a failed presidential campaign. And when Republicans took control of the chamber in 2015, McCain embraced his new job as chairman of the powerful Armed Services Committee, eager to play a big role “in defeating the forces of radical Islam that want to destroy America.”

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